Differential Solubility of Ethylene and Acetylene in Room-Temperature Ionic Liquids: A Theoretical Study

The room-temperature ionic liquids (RTILs) have potential in realizing the ethylene (C₂H₄) and acetylene (C₂H₂) separation and avoiding solvent loss and environmental pollution compared with traditional solvents. The interaction mechanisms between gases and RTILs are important for the exploration of new RTILs for gas separation; thus, they were studied by quantum chemical calculation and molecular dynamics simulation in this work. The optimized geometries were obtained for the complexes of C₂H₄/C₂H₂ with anions (Tf₂N^–, BF₄^–, and OAc^–), cation (bmim⁺), and their ion pairs, and the analysis for geometry, interaction energy, natural bond orbital (NBO), and atoms in molecules (AIM) was performed. The quantum chemical calculation results show that the hydrogen-bonding interaction between the gas molecule and anion is the dominant factor in determining the solubility of C₂H₂ in RTILs. However, the hydrogen-bonding interaction, the <i>p</i>–π interaction in C₂H₄–anion, and the π–π interaction in C₂H₄–cation are weak and comparable, which all affect the solubility of C₂H₄ in RTILs with comparable contribution. The calculated results for the distance of H_gas···X (X = O or F in anions), the BSSE-corrected interaction energy, the electron density of H_gas···X at the bond critical point (ρ_BCP), and the relative second-order perturbation stabilization energy (<i>E</i>(2)) are consistent with the experimental data that C₂H₂ is more soluble than C₂H₄ in the same RTILs and the solubility of C₂H₄ in RTILs has the following order: [bmim]­[Tf₂N] > [bmim]­[OAc] > [bmim]­[BF₄]. The calculated results also agree with the order of C₂H₂ solubility in different RTILs that [bmim]­[OAc] > [bmim]­[BF₄] > [bmim]­[Tf₂N]. Furthermore, the calculation results indicate that there is strong C₂H₂–RTIL interaction, which cannot be negligible compared to the RTIL–RTIL interaction; thus, the regular solution theory is probably not suitable to correlate C₂H₂ solubility in RTILs. The molecular dynamics simulation results show that the hydrogen bond between the H in C2 of the imidazolium cation and the anion will weaken the hydrogen-bonding interaction of the gas molecule and anion in a realistic solution condition, especially in the C₂H₄–RTIL system